Since July 1 high humidity and temperatures have been scorching Iowa. Okay, maybe that is a little extreme. June has led to dry conditions across much of central and southwest Iowa. Heat and dry weather are not desired weather conditions in the 2 weeks before or after pollination. This four-week time period is finalizing the kernel number per ear. Water use by the plant is at its peak of between 0.25 to 0.45 inches per day. That water is transpired as a cooling mechanism, but it is also needed for silk elongation and filling fertilized kernels.
What affect will heat and dry weather have? It is very hard to separate the impact of heat from drought stress because most of the time they occur simultaneously. Heat stress alone typically requires temperature of greater than 95oF in the absence of moisture stress. Temperature greater than 95oF along with low relative humidity can cause silk desiccation. Pollen is not killed until temperatures reach above 100oF, however, the pollen is likely damaged with temperature above the mid-90s. The good news is that pollination typically occurs in the mid-morning hours. From June 25 to July 7, temperatures in Iowa have not been greater than 95oF.
Moisture (or drought) stress is indicated by continual leave rolling and wilting. Because silks are very high in moisture content, moisture stress will slow down silk elongation. Silk elongation begins about 7 days before silks emerge from the ear husks. Severe moisture stress will reduce the number of kernels. It has been estimated that nearly continual wilting in the 2 weeks prior to silk emergence can reduce yield by 3-4% per day; during silk emergence and pollen shed by up to 8% per day; and in the two weeks following by up to 6% per day.
Below is a figure showing yield estimates at planting (May 1), June 1, July 1 and July 7 for seven locations across Iowa. These yield estimates are based on a crop model simulation (APSIM) that is used to generate the FACTS forecasts. These yield estimates use current weather up to July 7 and a 40-year weather history to finish out the growing season. Estimates of lost yield potential are greatest in central and southwest Iowa. Minimal impact (and maybe even a benefit) in northern and eastern Iowa has occurred from the May to July weather patterns. However, there is a large uncertainty on these yield estimates as indicated by the worst/best case estimates in the figure. Despite the heat and drought conditions, crops maintain high yield potential. In some areas of Iowa yield are trending down. The weather in the next two weeks is very critical. The remainder of July and August is still important in determining final yield.
Other modeling insights include:
- The water table depth has dropped to 6 feet on average from about 3 to 4 ft at planting.
- The 0 to 2 ft soil layer is very dry, the 2 to 4 ft layer has moisture, roots are about 4 to 4.5 ft.